1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an apparatus, which can maintain image density (gradation reproduction) of a color laser printer having a plurality of developers constant regardless of a individual difference of the apparatus or a mounting environment.
2. Description of the Related Art
For example, many users experience that copies obtained by copying the same original using the same copying machine have different densities. In an electrophotograph, an image density drift occurs under the influence of a change or deterioration of image forming conditions due to different environmental factors and an elapse of time. It is important for a multi-level printer or a digital copying machine as well as an analog copying machine to suppress and stabilize the image density drift. In particular, in a color image, since the image density drift influences not only density reproducibility but also color reproducibility, a stable image density is an indispensable requirement. Therefore, in a conventional apparatus, a given allowable margin is provided to image forming materials and an image forming process itself, and image stabilization is attained by maintenance within this allowable margin.
However, the allowable margin to be provided to the image forming materials and image forming process itself is limited, and the maintenance requires much labor and cost. Furthermore, the image density drift cycle is shorter than a maintenance cycle, and a stable image density cannot always be obtained by only the maintenance.
In this type of the digital copy apparatus, the relationship between the signal inputted from the image reader and density of an output image due to the printer is measured in advance, and a y characteristic, that is, a correlation between a reading characteristic of the image reader and an output characteristic of the printer can be obtained. Then, a correction parameter, which can provide an optimum y characteristic, is calculated. The calculated correction parameter is stored in a ROM (Read Only Memory), and is used to correct the signal inputted from the image reader in the form of a LUT (Look-up Table), thereby the gradation reproduction is improved.
However, in the conventional method, since the gradation reproduction cannot be adjusted under an image forming condition in which a developing characteristic is optimized by the correction parameter stored in LUT, texture is easily generated, and a favorable gradation expression and stability cannot maintained. Therefore, there is a problem in that the gradation reproduction cannot fully provided under the individual difference of the digital copy apparatus or the mounting environment.
In U.S. Pat. No. 4,975,745, (filed: Oct. 26, 1989), there is disclosed a method for forming two reference density patterns, measuring potentials of the respective patterns, and changing an image forming condition.